Support IPv6 and multiple IPs per interface

Hey!

Thanks for writing this :). It was missing a few features I need:

1. The ability to show all IP addresses for a given interface
2. Support for IPv6

I've tried to add support for a richer API `System.get_all_ifaddrs` while preserving the nice simplicity of `System.get_ifaddrs` for people who don't want something so complicated.

Please let me know if I need to fix the patch— I'm particularly unhappy about the tests, but I couldn't work out a nicer way :/

Conrad